1. Field of the Invention
The present invention relates to a DBF radar system employing as a receiving antenna a digital beam forming (DBF) antenna adapted to form a multiplicity of narrow beams, and more particularly to an improvement in interference resistant characteristics of a radar system.
2. Statement of Prior Art
Digital beam forming antenna systems are well known. The article title "DIGITAL MULTIPLE BEAM FORMING TECHNIQUES FOR RADAR" by Abraham E. Ruvin et al, EASCON--'78, pp 152-163 discloses such a receiving DBF antenna system in which signals received respectively by a plurality of antenna elements are connected to digital signals and then these digital signals are processed to simultaneously form a multiplicity of beams. U.S. Pat. No. 4,656,479 issued to Kirimoto et al on Apr. 7, 1987 titled "RADAR SYSTEM" discloses such a radar system including a transmitting section and receiving section, wherein electric waves transmitted from the transmitting section are received by a transponder and electric waves from the transponder are then received by a multiplicity of antenna elements of the receiving section, so that the signals received by the antenna elements are converted to digital signals which are in turn processed to determine the direction in which said received electric waves arrived.
In case of monitoring an object by use of a radar system employing such a digital beam forming antenna system, a problem is encountered in that the radar system may be easily affected by interference, should such an object be equipped with any means for generating interference. This possibility will now be explained with reference to FIG. 1.
FIG. 1 schematically illustrates the positional relationship between a radar apparatus 100 which employs a digital beam forming (DBF) antenna system and an interference apparatus 200 with which an object is equipped. It can be seen from FIG. 1 that a transmitting antenna 101 which forms a wide angle beam is connected to a transmitter 102 and serves to transmit into space an electric wave having the frequency of F.sub.1.
A DBF receiver 103 includes a plurality of antenna elements 104 which are arranged in one dimension or two dimensions. The respective antenna elements 104 supply the received electric waves to receivers 105. The signals output from the respective receivers 105 are then converted to digital signals by A/D converters 106. A plurality of digital signals are simultaneously transferred from the respective A/D converters 106 and input to a beam former (an operating means) 107 adapted to effect DFT (Digital Fourier Transform), which discriminates the received electric waves with respect to the incoming directions and supplies outputs in accordance with the respective particular directions. A non-scanning DBF antenna system for a mulitplicity of narrow beams is thus made by the above-described components from the antenna elements 104 through the beam former 107 and may be used as a receiving antenna.
The interference apparatus 200 situated on the object includes a radar wave analyzing device 201 which is called an ESM (Electronic Support Measures) and an interference wave generating device 202 which is called an ECM (Electronic Counter Measure). ESM 201 is connected to a receiving antenna 203, while ECM 202 is connected to a transmitting antenna 204. The radar wave analyzing device 201 includes a multichannel receiver 205 which receives only a predetermined electric wave out of the electric waves received by the receiving antenna 203, a signal characteristics extracting device 206 adapted to extract characteristics (such as pulse duration, frequency, pulse amplitude, etc.) of the signals output from the multichannel receiver 205 and a signal analyzing device 207 which analyzes the signals in accordance with the characteristics output from the signal characteristics extracting device 206. The interference wave generating device 202 generates interference waves equivalent to the received radar waves based on the information received from the signal analyzing device 207 and transmits the interference waves through the transmitting antenna 204.
At the side of the DBF radar 100, an electric wave having a frequency of Fi is transmitted over a wide range by the use of the wide angle beam transmitting antenna 101. The reflection echo which has been reflected by the object is received by the DBF receiver 103. The reflected echo is received by the beam former 107 by way of the antenna element 104, the receiver 105 and the A/D converter 106. The beam former 107 effects the Fourier Transform on the digitized signals and simultaneously forms a plurality of beams. The received electric waves are thus discriminated and output with respect to each of the incoming directions to provide information on the position, speed, etc. of the respective objects. The DBF receiver 103 is capable of forming a plurality of narrow beams at the same time and classifying a plurality of reflected echos of the frequency F.sub.1 received simultaneously from different directions in accordance with the directions in which the echos arrive.
At the side of the object, the channelized receiver 205 receives the radar wave through the receiving antenna 203. The signal characteristics extracting device 206 extracts the characteristics of the received radar wave which is in turn analyzed in accordance with the pulse duration, frequency, pulse amplitude, etc. of the output from the signal analyzer 207. Subsequently, when the radar wave has been analyzed, the interference wave generating device 202 generates an interference wave equivalent to the received radar wave and transmits the interference wave to the DBF radar 100 by way of the transmitting antenna 204.
Since the DBF radar 100 transmits an electric wave having the specific frequency F.sub.1 over a wide range, the transmission of such an electric wave may be easily detected and analyzed by the ESM of the opponent and, as a consequence, the DBF radar 100 is readily interfered by the electric wave having an identical frequency F.sub.1.